Recently, water base paints have been extensively developed in many countries including Japan as the replacement of the organic solvent base paints for saving costs and resolving the environmental pollution problems. In this connection, the development of metallic pigments suitable for incorporating in the water base paints has been highly desired to use for top coating the automobiles and the other vehicles so as to give them the high grade appearances.
As the metallic pigment for incorporating in the water base paint, a pigment composition comprising aluminium flakes and a surfactant which is added for improving the water dispersibility has been known. However, this composition has the important defect of evolving a large volume of hydrogen gas during a long storage period, thereby an explosion may occur.
As the method for resolving the above-mentioned defect, the methods of forming anti-corrosive coatings on the aluminium flakes by adding an anti-corrosive material such as morpholine, dimeric acid, organic phosphate ester, boric acid salt and the like in addition to the surfactant have been proposed. However, these methods could not completely prevent the evolution of hydrogen gas. Further, the coatings formed on the aluminium flakes caused another defects of darkening the color tone characteristic of the metallic pigment as well as aggregating the flakes during the long storage period.
A method of contacting aluminium flakes with an aqueous solution containing available phosphate ions supplied from the phosphate compound such as (NH.sub.4).sub.2 HPO.sub.4 and the like in an amount of at least 0.7 % by weight based on the aluminium weight and having pH of about 1.5 to 11 is disclosed in U.S. Pat. No. 2,858,230. A pigment composition obtained by the above-mentioned method can prevent considerably the evolution of hydrogen gas in the aqueous medium, but this composition cannot give the paint film with the satisfactory color tone. And, this composition tends to gradually darken the color tone with the evolution of the gas and to aggregate the flakes during the storage.
An object of the invention is to provide a pigment composition which can be incorporated in the water base paint without showing the above-mentioned defects.
An object of the invention is to provide an aluminium flake pigment composition which when incorporated in the water base paint, evolves a little volume of hydrogen gas and has a little tendency of darkening the color tone and aggregating the flakes during the long storage period.